Sensors that detect a measured quantity essential for controlling internal combustion engines, such as the air drawn in by the internal combustion engine, should be monitored to detect malfunctions in a reliable manner. One way to determine a malfunction is by implementing a signal-range check, for example. In such a signal-range check or signal-amplitude analysis, the output signal of the sensor is monitored to determine whether it lies within a plausible range. A signal-range check may be performed, for example, by determining whether the output signal of a sensor lies between an upper and a lower limit. If the output signal exceeds the upper limit or undershoots the lower limit, a malfunction is determined and displayed.
A signal-range check for monitoring an air-mass flow sensor of an internal combustion engine is referred to, for example, in European Published Patent Application No. 0 778 406. The malfunction detection method referred to not only performs a signal-range check on the basis of fixed limits, but also determines a permissible range for the output signal of the air-mass flow sensor as a function of the opening angle of the throttle valve. As the opening angle of the throttle valve increases, so does the output voltage of the air-mass sensor regarded as plausible. If the output signal lies outside the plausible range, a fault is detected and the output signal is no longer considered in further signal analysis.